Scope in NodeJS / Firebase promise - node.js

Question

I'm trying to develop a NodeJS app connecting to Firebase. I can connect successfully, but I'm unable to figure how to manage the scope in the then call.
I'm using NodeJS 6.9.2
My test implementation looks like this:
const EventEmitter = require('events');
const fb = require('firebase')
class FireGateway extends EventEmitter {
constructor() {
super();
if ( this.instance ) {
return this.instance;
}
// INIT
var fbConfig = {
apiKey: "xxxxx",
authDomain: "xxxxx.firebaseapp.com",
databaseURL: "https://xxxxx.firebaseio.com/"
};
fb.initializeApp(fbConfig)
this.instance = this;
this.testvar = "aaa";
}
login() {
fb.auth().signInWithEmailAndPassword ("email", "pwd")
.catch(function(error) {
// Handle Errors here.
}).then( function(onresolve, onreject) {
if (onresolve) {
console.log(this.testvar);
// "Cannot read property 'testvar' of undefined"
this.emit('loggedin');
// error as well
}
})
}
}
module.exports = FireGateway;
------
...
var FireGateway = require('./app/fireGateway');
this.fireGW = new FireGateway();
this.fireGW.login();
....
Any idea how can I manage it?

Answer 1

The callback passed to then is being called asynchronously from another context, so the this doesn't correspond to the instantiated object.
Using ES6 arrow functions you can keep your object context, since an arrow function does not create its own this context.
By the way, the syntax you are using in the then method is not correct, then accepts two callbacks with one argument each one. Check the syntax here.
The catch before the then is not necessary as well I think, it would make more sense to put it at the end.
It would be something like this:
login() {
fb.auth().signInWithEmailAndPassword("email", "pwd")
.then(
(onResolve) => {
console.log(this.testvar);
this.emit('loggedin');
},
(onReject) = > {
// error handling goes here
});
}
On the other hand, it seems login method is doing an asynchronous operation, so you might want to wait for it to finish in your code. I would make the login method return a Promise, so you can wait for it outside:
login() {
return fb.auth().signInWithEmailAndPassword("email", "pwd")
...
}